I had that happening the other day, after ordering to go very deep...despite all the creaking sounds, our sub seemed ok...no warning whatorever.
Then I suddenly got the feeling I had to check the conning-tower.
Went upthere, and found lights flickering, and steam emerging from some panel.
Blow ballast!!!
And we weren't even near crush-depth.
And the next day, I tried the same sub, and could go beyond it, with no problems. :hmm: